The way that any point on a surface interacts with light can be described by its Bidirectional Reflectance Distribution Function, or BDRF. This function is a mapping from the two dimensions of incoming light direction to the two dimensions of outgoing light direction, or a mapping from (u,v) to (u′,v′). In order to create visually realistic computer graphic simulations of complex real-world surfaces, such as wood or woven fabric or human skin, it is useful to measure the actual BRDF of such surfaces. For example, once the BRDF of small patches of skin on a human face have been measured, then the surface of an entire new face can be synthesized by seamlessly patching together such samples. The visually realistic synthesis of large areas of textured surfaces from small example patches is well known in the literature [A. Efros and W. Freeman. Image Quilting for Texture Synthesis and Transfer. Proceedings of SIGGRAPH '01, Los Angeles, Calif., August, 2001, incorporated by reference herein].
One bottleneck to this process is the need to measure the BRDF of real-world samples. Current techniques to do this are highly invasive, in that they require the sample to be placed in a specially lit environment [S. Marschner, S. Westin, E. Lafortune, K. Torrance, and D. Greenberg. Image-based BRDF Measurement Including Human Skin. In 10th Eurographics Workshop on Rendering, pages 131–144, June 1999, incorporated by reference herein]. For some surfaces, such as living human skin, which cannot be placed by itself in an isolated measuring chamber, this is a difficult, tedious and expensive process.
The following is a description of a device to quickly and accurately measure the BDRF of a sample region of a surface in situ. The device can be made small and portable, requires no moving parts, and can be used in any lighting situation.
The new technique requires no physical movement between sub-measurements, thereby guaranteeing that all sub-measurements will be perfectly registered with one another. This property allows an improvement in accuracy in comparison with previous methods for measuring BRDF that require physical movement between sub-measurements.
Also, the new technique requires only a single CCD camera or equivalent image capture device. This property allows the device to be fabricated at a low cost in comparison with previous methods that require multiple CCD cameras or equivalent image capture devices.
All of these qualities make the new method a valuable measurement tool for use in situations for which current techniques are too bulky or unwieldy. For example, during a motion picture production, a computer graphics special effects expert could use a device employing the new method to measure the response to light of the skin of various parts of an actor's face, or the fabric of a costume, or a prop or other part of the set. With this information in hand, then through the use of currently known techniques in computer graphics synthesis [P. Hendrik, J. Lansch, M. Goesele, W. Heidrich and H. Seidel. Image-Based Reconstruction of Spatially Varying Materials. In Twelfth Eurographics Rendering Workshop 2001, pages 104–115, Eurographics, June 2001, incorporated by reference herein], the appearance of these items can then be duplicated digitally with highly convincing realism and fidelity.